The present invention is directed generally to gas generating compositions, methods of production of the same and devices made therefrom and, more particularly, gas generating compositions having varying burn rates catalyzed using borohydride salts for use in seat belt pretensioners and other applications requiring rapid gas generation.
Gas generating compositions have been used for many years in various pyrotechnic applications. In recent years, gas generating compositions have been found to be useful in safety applications, such as in vehicle passive restraint airbag systems and seat belt pretensioners.
The new application of gas generating technology to consumer products have raised new issues relating to the exposure of consumers to the technology. For example, in the context of automobile safety restraint systems, gas generating compositions must satisfy several important design criteria. The design criteria require that gas generated by reacting the compositions be generated almost instantaneously and at relatively low temperatures to minimize the potential for burning the automobile occupants. The safety restraint specifications also put strict limits on the generation of toxic or harmful gases and solid particulates.
Currently, the most commonly employed gas generant in automotive safety restraint systems is sodium azide (NaN.sub.3), which by itself is a relatively toxic material. The oral rat LD.sub.50 of NaN.sub.3 has been reported as 27 mg/kg.
The combustion products of the sodium azide gas generant are also considered to be relatively toxic. In most of the safety restraint systems using sodium azide as the gas generant, molybdenum disulfide or sulfur has been utilized as oxidizers for the sodium azide. The gas generant reaction products include hydrogen sulfide, sodium hydroxide, and sodium sulfide which are all fairly caustic.
A number of efforts have been initiated to develop replacements for the azide fuel gas generant compositions in vehicle restraint systems. The development are generally focused on replacing the azide fuel, and particularly NaN.sub.3, with fuel and oxidizer compositions that produce more benign oxidation products, specifically N.sub.2, H.sub.2 O, and CO.sub.2. However, the various non-azide compositions have not been extensively used to date as replacements for azide composition systems.
The azide free fuel compositions generally employ a blend of two or more discrete fuel sources, such as tetrazoles and triazoles, dicyanamide salts, and other nitrogen containing compounds in an attempt to provide performance comparable to azide fuels. The discrete fuel sources are mixed with one or more oxidizers, such as transition metal oxides, nitrates, chlorates and perchlorates, in varying quantities to produce a desired gas generation rate. The compositions also may include catalysts and binders for additional control over the burn rate and for processing, respectively.
In both the azide and non-azide systems, the performance predictability of the compositions depends upon the homogeneous distribution of the discrete fuel source and the discrete oxidizer within the compositions. Therefore, it is important that the composition ingredients be mixed to a sufficient extent to ensure the homogeneity of the mixture. However, in practice, it is improbable that homogeneous mixtures will actually be achieved, especially as batch size of the composition increases.
Recognizing the practical inhomogeneity of the compositions, one method of ensuring the proximity of the fuel sources to the oxidizers is to provide an excess amount of the oxidizer. For example, oxidizers are often include 200% of the stoichiometric quantity needed to completely oxidize the fuel, and the resulting percentage of the fuel in the composition often ranges from 10-20%. The excess oxidizer increases the amount of the composition necessary for a specific application and the overall cost of the composition.
Thus, there are continuing needs for gas generating compositions and safety devices produced therefrom that are less costly, more predictable in performance, and more compatible with consumer related applications, such as airbags and seat belt pretensioners.